U.S. space scientists have moved one step closer to determining whether Mars once harbored life. Earlier this month, one of two robotic vehicles roaming the planet found evidence that water had once eroded stone. Now they've announced that the evidence points to something even more important.

Washington, 24 March 2004 (RFE/RL) -- American scientists say they have found the strongest evidence yet that Mars was once hospitable to life.

Speaking with reporters yesterday in Washington, officials from the U.S. space agency NASA were careful to say they had not found evidence of life itself. But, they say, "Opportunity" -- one of two robotic vehicles that landed on the planet in January -- has found evidence of a shallow, salty sea.

The conclusion was based on the same evidence that led to the announcement on 2 March that water had, indeed, once flowed on Mars. At that time, however, scientists were able to conclude only that it was present in the Meridiani Planum, the area being explored by one of the vehicles.

"We don't know that life was there, but we have an environment that would have been suitable for life."

Ed Weiler, NASA's associate administrator for space science, said early analysis of the evidence indicates that some Martian rocks had undergone erosion that could only have been caused by water. He said further study determined something even more important.

"It appears that the rocks at Meridiani were not just altered or modified by water, they were actually formed in water, perhaps a shallow, salty sea," he said. "This is a profound discovery. It has profound implications for astrobiology [the study of extraterrestrial life]. And I'd like to say, if you have an interest in searching for fossils on Mars, this is the first place you want to go."

Steve Squyres, the main scientist for the Mars mission, said the fact that the rock was formed in water, rather than merely eroded by water, indicates the onetime presence of enough water to constitute a sea.

"We have found what I believe to be strong evidence that the rocks themselves are sediments that were laid down in liquid water. It's a fundamental distinction. It's like the difference between water you can draw from a well and water you can swim in."

In other words, Squyres says, a mere trickle of water would not be enough to sustain life like that known on Earth. But he said the Martian sea in the Meridiani Planum was plentiful and salty.

"These are the kinds of environments that are very suitable for life. Now, we don't know that life was there, but we have an environment that would have been suitable for life. The second reason it's important is the potential for preservation of evidence."

Squyres said the minerals in the water may have trapped whatever was submerged in it, perhaps leaving fossils for eventual discovery.

There is still a lot that scientists do not know about Mars and its water, Squyres says -- for example, the size of the newly discovered sea, and how common such seas may have been on the planet.

They also do not know the size of the former body of water found by the Mars rover, whether it was once a permanent sea, or whether it flooded from time to time like a desert basin. There is also no evidence yet as to when this water existed, or how long the area was wet.

But Squyres said that uncovering the evidence of the salty sea on Meridiani Planum proves that NASA has the means to find answers to at least some of these questions.

The U.S. space agency plans to mount unmanned missions to the planet every 26 months. One such mission, scheduled for 2013, would return to Earth with rock samples for more thorough analysis.

U.S. President George W. Bush also has proposed sending humans to Mars, although he has not given a detailed schedule for that effort.